JP2006058508A - Optical switch - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical switch of which the operation characteristics are not varied and prevented from being affected by polarization cross talk even when a wavelength of signal light and surrounding temperature of the optical switch are varied. <P>SOLUTION: The optical switch is equipped with first and second polarization separation/synthesis modules 10, 18, first and second polarization plane preserving optical fibers 12, 16, a first polarization plane conversion part 14, third and fourth polarization plane preserving optical fibers 22, 26 and a second polarization plane conversion part 24. An end of the first polarization plane preserving optical fiber is bonded to a second input/output terminal 10-2, an end of the second polarization plane preserving optical fiber is bonded to a first input/output terminal 18-1, and the other ends of the first and second polarization plane preserving optical fibers are connected to each other via the first polarization plane conversion part. Also the third polarization plane preserving optical fiber is bonded to a second input/output terminal 18-2 at an end thereof and further is equipped with an optical coupler 20. The fourth polarization plane preserving optical fiber is bonded to a third input/output terminal 18-3 at an end thereof, and the other ends of the third and fourth polarization plane preserving optical fibers are connected to each other via the second polarization plane conversion part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical switch used for long-distance large-capacity optical fiber communication or the like that switches controlled light with control light.

  In order to effectively use limited communication line resources and realize large-capacity optical fiber communication, means for increasing the number of channels that can be transmitted and received and increasing the communication speed are required.

  As one means for increasing the number of channels, a multiplex communication method such as Time Division Multiplexing (TDM) has been studied. The TDM is transmitted as a time-division multiplexed signal obtained by time-division-multiplexing a plurality of channels, and each channel is separated by demultiplexing means for separating each channel from the time-division multiplexed signal by a gate signal generated from the clock signal on the receiving side. This is a communication method that employs a method of individually retrieving and receiving the information.

  In order to increase the communication speed of the TDM described above, it is desirable to realize all the demultiplexing means by optical means. That is, it is possible to realize an optical switch that can execute a switch operation for blocking / transmitting an optical pulse constituting an optical pulse signal that is a controlled light by using only the optical control signal that is a control light without using an electrical means. desirable.

  The optical Kerr effect that appears in an optical fiber is a phenomenon in which the refractive index of the optical fiber changes due to the propagation of strong light through the optical fiber. The response speed of the optical Kerr effect is several femtoseconds (fs). That is, if an optical switch is configured using the optical Kerr effect, an optical switch capable of switching an optical pulse signal of several hundred Gbit / s or more may be realized. By the way, with a conventional switch that converts an optical pulse signal into an electrical pulse signal that is an electrical signal, switches the electrical pulse signal back to an optical pulse signal after switching with an electronic device, a bit rate of about 40 Gbit / s It was the limit to switch the optical pulse signal.

  As an optical switch using the optical Kerr effect, an optical switch using the optical Kerr effect developed in a polarization-maintaining single-mode optical fiber has been studied (for example, see Non-Patent Document 1).

  An optical switch using the optical Kerr effect disclosed in Non-Patent Document 1 is a polarization-maintaining single-mode optical fiber (hereinafter referred to as “polarization plane-maintaining optical fiber” or simply “optical fiber”). Is sometimes used.). A polarization maintaining optical fiber is a slow axis or slow axis set in a plane perpendicular to the light propagation direction of this fiber (hereinafter sometimes referred to as the “optical axis direction of the optical fiber”). The direction of the optical axis called “a” and the direction of the optical axis called the fast axis or the fast axis orthogonal to the slow axis have a structure in which the equivalent refractive index with respect to the guided light is different.

  That is, a stress applying portion having a refractive index higher than the refractive index of the clad is disposed near the core of the optical fiber, and the equivalent refractive index for light in which the vibration direction of the electric field vector of the light is parallel to the slow axis direction is Is designed so that the oscillation direction of the electric field vector is higher than the equivalent refractive index for light parallel to the direction of the fast axis. Due to the asymmetry of the equivalent refractive index, the polarization plane of the light input to the polarization plane preserving optical fiber is stored and propagated. Hereinafter, the vibration plane of the linearly polarized electric field vector is referred to as a polarization plane.

  The optical fiber used in the optical switch disclosed in Non-Patent Document 1 has a surface fused with the optical axes of two polarization-maintaining single-mode optical fibers orthogonal to each other. It has a structure capable of canceling the birefringence of the storage type single-mode optical fiber. This optical switch includes linearly polarized control light having a polarization plane parallel to the optical axis of the polarization-maintaining optical fiber, and linear polarization having a polarization plane inclined by 45 ° from the optical axis of the polarization-maintaining optical fiber. Signal light is input.

  When the optical pulse that constitutes the signal light and the optical pulse that constitutes the control light are not input to this optical switch in synchronization, the optical pulse of the signal light is in the same linear polarization state as when input to this optical switch. Is output. On the other hand, when the optical pulse of the control light and the optical pulse of the signal light are input in synchronization, the polarization component parallel to the polarization direction of the optical pulse of the control light among the polarization components of the optical pulse of the signal light On the other hand, the optical Kerr effect is induced by the light pulse of the control light. That is, the optical Kerr effect causes a phase shift in the optical pulse of the signal light due to the cross-phase modulation effect that appears between the optical pulse of the signal light and the optical pulse of the control light.

The above-described phase shift φ is given by the following equation (1).
φ = 2γPL (1)
Here, P (W) is the power of the control light, and L (km) is the length of the optical fiber constituting the optical fiber loop. γ (W ⁻¹ km ⁻¹ ) is a nonlinear optical constant based on the optical Kerr effect. γ against the (W ^-1 km ^-1) normal optical fiber, 1-2 although W is a value of about ^-1 miles ^-1, several tens to several hundreds W ^-1 miles ^-1 value of about A special optical fiber called a highly nonlinear optical fiber with a reduced effective area has also been developed.

  When the phase shift amount φ is equal to π, the polarization direction of the optical pulse of the signal light is rotated by 90 ° with respect to the input time to the optical switch. That is, the polarization direction of the optical pulse of the signal light is −45 ° with respect to the optical axis of the optical fiber. By arranging the analyzer on the output side of the optical switch, the optical pulse of the signal light can be passed or blocked by the control light. That is, the direction of the optical axis of the analyzer is transmitted when the polarization direction of the optical pulse of the signal light is rotated by 90 ° with respect to the input to this optical switch, and the same polarization direction as that at the input In such a case, if it is arranged in the blocking direction, only the optical pulse whose polarization plane is rotated by the control light can pass through this optical switch, so that the optical pulse of the signal light can be switched by the control light.

  In the optical switch disclosed in Non-Patent Document 1, in order for the above switching operation to be performed steadily, the polarization state of each optical pulse of the signal light and the control light is maintained in the polarization maintaining optical fiber. It is a prerequisite. This optical switch has a structure in which the optical axis of the polarization-maintaining single-mode optical fiber is orthogonally fused at an intermediate position in the optical axis direction of the polarization-maintaining optical fiber (sometimes referred to as a “fused portion”). ) Is devised to substantially maintain the polarization state of the light pulse.

  More specifically, the fact that the polarization state of the light pulse is substantially maintained is as follows. First, the polarization-maintaining optical fiber in the first stage extends from the incident end to the fused portion of the polarization-maintaining optical fiber, and the second-stage polarization-maintaining light extends from the fused portion to the output end of the polarization-maintaining optical fiber. It will be called a fiber. Linearly polarized light with a 45 ° polarization plane tilted with respect to the optical axis of the first-stage polarization-maintaining optical fiber is input. At this time, components parallel to the fast axis and the slow axis of the first-stage polarization-maintaining optical fiber of the input light are defined as an S component and a P component, respectively. It is assumed that the phase difference generated between the S component and the P component of the input light in the first-stage polarization-maintaining optical fiber is φ.

  At this time, the optical switch is configured so that the phase difference generated between the S component and the P component of the input light in the second-stage polarization plane-maintaining optical fiber becomes −φ. This means that a polarization maintaining optical fiber is designed. In other words, the signal light is input to the path from the input end of the first-stage polarization-maintaining optical fiber, which is the input end of the optical switch, to the analyzer. The above-mentioned fused portion is provided at a position where the optical path length (the geometric length multiplied by the refractive index) matches when input as polarized light and when input as TE polarized light. It is.

  That is, in order to realize the above-described operation, it is ensured that the first-stage polarization-maintaining optical fiber and the second-stage polarization-maintaining optical fiber have the same structure. Therefore, it is necessary to set the lengths of the first-stage polarization-maintaining optical fiber and the second-stage polarization-maintaining optical fiber to be equal to each other.

Panda (PANDA: Polarization-maintaining AND Absorption-reducing) type optical fiber, which is widely used as a polarization-maintaining optical fiber, is effective refraction when the direction of oscillation of the electric field vector of the guided light is parallel to the fast axis. The difference between the refractive index and the effective refractive index when parallel to the slow axis is about 3 × 10 ⁻⁴ . Therefore, in the case where the wavelength of the guided optical pulse is 1.5 μm, for example, if the optical pulse propagates about 5 mm through this polarization-maintaining optical fiber, the vibration direction of the electric field vector is a component parallel to the fast axis. And a phase difference of 2π is generated between the vibration direction of the electric field vector and a component parallel to the slow axis.

  In other words, if the difference in length between the first-stage polarization-maintaining optical fiber and the second-stage polarization-maintaining optical fiber cannot be set sufficiently smaller than 5 mm, this polarization-maintaining optical fiber is used. This means that the propagating state of the propagating light pulse cannot be substantially maintained.

  In general, the length of an optical fiber used for this type of optical switch is several tens of meters to several hundreds of kilometers, and the total length of such an optical fiber is set by a system of mm or less. That is very difficult. If the difference in length between the first-stage polarization-maintaining optical fiber and the second-stage polarization-maintaining optical fiber is not set sufficiently small, the ambient temperature, signal light, or control of this optical fiber The characteristics as an optical switch cannot be maintained with respect to fluctuations in the wavelength of light.

  In addition, in a polarization maintaining optical fiber such as a commercially available PAND type optical fiber, the direction of the fast axis (or slow axis) is not completely invariant along the longitudinal direction. Therefore, even if the input light to the polarization-maintaining optical fiber is linearly polarized light whose polarization plane is parallel to the fast axis (or slow axis) of the polarization-maintaining optical fiber, The output light has a polarization component in a direction orthogonal to the polarization direction of the input light. The component in the polarization direction orthogonal to the polarization direction of the input light is called polarization crosstalk.

  In commercially available PANDA type optical fibers with average polarization preserving performance, this polarization crosstalk is known to increase rapidly when the length of the PANDA type optical fiber is several tens of meters or more. (For example, see Non-Patent Document 2).

As already explained, an optical switch using the optical Kerr effect is usually configured using a polarization-maintaining optical fiber having a length of several tens of meters or more. Need to be considered. An optical pulse propagating through the polarization-maintaining optical fiber constituting the optical switch has polarization components in both the fast axis and slow axis directions. Therefore, if polarization crosstalk occurs, the original polarization direction component of the optical pulse of the signal light interferes with the polarization crosstalk, and the polarization state of the optical pulse of the signal light does not exist. It becomes a different state. The effect given to the optical pulse of the signal light by this polarization crosstalk also varies depending on the wavelength of the optical pulse of the signal light, the ambient temperature of the polarization plane preserving optical fiber, and the like. That is, the presence of polarization crosstalk causes a problem of causing an unstable operation that varies the operating characteristics of the optical switch.
"Ultrafast optical multi / demultiplexer utilising optical Kerr effect in polarisation-maintaining single-mode fibers," T. Morioka, M. Saruwatari and A Takada, Electronic Letters, vol. 23, No. 9 pp. 453-454, 1987. "Polarization-maintaining optical fiber" Arai, Saito, Oyama, Nakamura, Yokomizo, Aiso, Furukawa Electric Times, 109, pp. 5-10, 2002.

  Therefore, the object of the present invention is to operate stably even if the wavelength of the signal light that is the controlled light and the ambient temperature of the optical switch change, the operation characteristics do not change, and the operation is not affected by the polarization crosstalk. Is to provide a guaranteed optical switch.

  To achieve the above object, an optical switch of the present invention includes a first polarization separation / combination module, a second polarization separation / combination module, a first polarization plane maintaining optical fiber, and a second polarization plane maintaining optical fiber, A first polarization plane converting unit, a third polarization plane preserving optical fiber, a fourth polarization plane preserving optical fiber, and a second polarization plane converting unit.

  The first polarization separation / combination module includes a first input / output end for inputting signal light, a second input / output end facing the first input / output end for coupling one end of the first polarization-maintaining optical fiber, And a third input / output terminal for outputting the switched signal light.

  The second polarization separation / combination module combines the first input / output end for coupling the other end of the second polarization-maintaining optical fiber, and one end of the third polarization-maintaining optical fiber on the side facing the first input / output end. A second input / output end, a third input / output end that couples one end of the fourth polarization-maintaining optical fiber, and a fourth input / output end that outputs a polarization crosstalk component to the side facing the third input / output end It has.

  One end of the first polarization maintaining optical fiber is coupled to the second input / output end of the first polarization separation / combination module, and the second polarization maintaining optical fiber is connected to the first input of the second polarization separation / combination module. The other end is coupled to the output end, and the other end of the first polarization-maintaining optical fiber and one end of the second polarization-maintaining optical fiber are connected via a first polarization plane converter.

  The third polarization-maintaining optical fiber has one end coupled to the second input / output end of the second polarization separation / combination module and includes an optical coupler. The fourth polarization-maintaining optical fiber is the second polarization-maintaining optical fiber. The other end of the third polarization-maintaining optical fiber and the other end of the fourth polarization-maintaining optical fiber are connected via the second polarization plane converter. Connected.

  First, light is incident on a reflection surface having polarization plane selectivity of the polarization separation / combination element (hereinafter, also referred to as “polarization plane selection reflection surface”) that constitutes the first polarization separation / combination module. In this case, the component corresponding to the vibration direction of the electric field vector with respect to the polarization plane selective reflection surface of the incident light is defined as follows. That is, the component that the electric field vector oscillates in the direction parallel to the incident surface of the incident light incident on the polarization plane selective reflection surface of the polarization splitting / combining element is the p component, and the electric field vector oscillates in the direction perpendicular to the incident surface of the incident light. The component to be called is referred to as the s component.

  According to the optical switch of the present invention, the linearly polarized signal light that is the input signal light to the optical switch is input to the first input / output end of the first polarization separation / combination module, and the polarization plane of the signal light is Stored and output from the second input / output terminal of the first polarization separation / combination module.

  In order to operate in this way, the polarization direction of the input signal light is set as follows. That is, linearly polarized signal light that is input signal light to the optical switch is input to the first input / output terminal of the first polarization separation / combination module as p-polarized light. The polarization plane selective reflection surface of the polarization beam splitting / combining element has a property of transmitting only the p component and reflecting only the s component. Therefore, the signal light input as p-polarized light to the first input / output end of the first polarization separation / combination module remains p-polarized light and faces the first input / output end of the first polarization separation / combination module. Is output from the second input / output terminal installed in the.

  The signal light output from the second input / output end of the first polarization separation / combination module is such that the plane of polarization coincides with the optical axis of the first polarization plane preserving optical fiber from one end of the first polarization plane preserving optical fiber. Is transmitted to the first polarization plane preserving optical fiber, reaches the first polarization plane converter installed at the other end of the first polarization plane preserving optical fiber, passes there, and is input to one end of the second polarization preserving optical fiber. The

  The first polarization plane conversion unit is formed, for example, by fusing the optical axis of the first polarization plane preserving optical fiber and the optical axis of the second polarization plane preserving optical fiber at an angle of 45 ° with each other. Yes. Therefore, as will be described in detail later, the signal light that has passed through the first polarization plane conversion unit has a polarization state having an electric field vector component parallel to the optical axis of the second polarization plane preserving optical fiber and a vertical electric field vector component. Then, it propagates through the second polarization-maintaining optical fiber and is input to the first input / output end of the second polarization separation / combination module. The signal light input to the second polarization separation / combination module via the first input / output end of the second polarization separation / combination module is converted into the first signal light and the second signal light whose polarization planes are orthogonal to each other. Separated and output from the second input / output end and the third input / output end of the second polarization separation / combination module, respectively.

  The first signal light is input from one end of the third polarization-maintaining optical fiber, propagates through the third polarization-maintaining optical fiber, and is provided at the other end of the third polarization-maintaining optical fiber. It passes through the converter and is input to the fourth polarization-maintaining optical fiber. The second polarization plane conversion unit is formed, for example, by fusing the optical axis of the third polarization plane preserving optical fiber and the optical axis of the fourth polarization plane preserving optical fiber at an angle of 90 ° with each other. ing. Therefore, the polarization plane of the first signal light that has propagated through the third polarization-maintaining optical fiber is rotated by 90 ° and input to the fourth polarization-maintaining optical fiber. Thus, the first signal light propagated through the fourth polarization-maintaining optical fiber is input to the third input / output terminal of the second polarization separation / combination module.

  On the other hand, the second signal light is output from the third input / output end of the second polarization separation / combination module, input from one end of the fourth polarization-maintaining optical fiber, and propagates through the fourth polarization-maintaining optical fiber. Then, the light passes through the second polarization plane converter provided at the other end of the fourth polarization plane-maintaining optical fiber and is input to the third polarization plane-maintaining optical fiber. As in the case of the first signal light, the polarization plane of the second signal light propagated through the fourth polarization-maintaining optical fiber is rotated by 90 ° and input to the third polarization-maintaining optical fiber in the second polarization plane conversion unit. Is done. Thus, the second signal light propagated through the third polarization-maintaining optical fiber is input to the second input / output terminal of the second polarization separation / combination module.

  Here, the direction of the polarization plane of the second signal light input to the second input / output end of the second polarization separation / combination module and the second polarization via the first input / output end of the second polarization separation / combination module. Of the signal light input to the wave separation / combination module, the direction of the polarization plane of the first signal light output to the second input / output terminal of the second polarization separation / combination module coincides. The direction of the polarization plane of the first signal light input to the third input / output end of the second polarization separation / combination module and the second polarization via the first input / output end of the second polarization separation / combination module Of the signal light input to the separation / combination module, the direction of the polarization plane of the second signal light output to the third input / output terminal of the second polarization separation / combination module coincides.

  Therefore, after the second signal light and the first signal light combined by the second polarization separation / combination module are combined, the signal light has been input to the second polarization separation / combination module 18 Propagating from the second polarization-maintaining optical fiber toward the first polarization-maintaining optical fiber, in the opposite polarization direction when the signal light is input in exactly the same polarization state, The signal is input from the second input / output end of the single polarization splitting / combining module and output from the first input / output end opposite to the second input / output end.

  That is, when the linearly polarized signal light input to the first input / output end of the first polarization separation / combination module is input from the first input / output end of the first polarization separation / combination module, as described above, Output in the same polarization state. In other words, the signal light input to this optical switch is reflected and can be regarded as being output from the first input / output end of the first polarization separation / combination module. The signal light output from the first input / output terminal of the module is sometimes referred to as loop reflected light.

  Here, consider a case where control light is input from an optical coupler provided in the third polarization-maintaining optical fiber. In this case, the phase velocity of the first signal light propagating through the third polarization-maintaining optical fiber changes. When the length of the third polarization-maintaining optical fiber is adjusted according to the intensity of the control light, and when the control light is present or not when the first signal light has propagated through the third polarization-maintaining optical fiber And the phase difference can be set exactly equal to π.

  If so configured, the details will be described later, but after the second signal light and the first signal light are combined in the second polarization separation / combination module, the first polarization plane conversion unit is changed to the second polarization plane. When passing from the storage optical fiber toward the first polarization-maintaining optical fiber, the signal light passes from the first polarization-maintaining optical fiber to the second polarization-maintaining optical fiber as the first polarization as input signal light to this optical switch. It propagates through the first polarization plane preserving optical fiber as output signal light having a polarization plane orthogonal to the direction of the polarization plane when passing through the wavefront converter. Therefore, this output signal light is input from the second input / output end of the first polarization splitting / combining module and output from the third input / output end. Hereinafter, the signal light output from the third input / output terminal of the first polarization separation / combination module may be referred to as loop transmitted light.

  As described above, the optical switch of the present invention has a common path for the first signal light and the second signal light, so even if the wavelength of the signal light or the ambient temperature of the optical switch changes, It is possible to realize an optical switch whose operation characteristics are not changed and whose stable operation is guaranteed.

  In this optical switch, the polarization crosstalk mainly occurs in the third polarization-maintaining optical fiber having the longest fiber length. Since the polarization crosstalk component mixed in the first signal light is a component having a polarization direction orthogonal to the polarization direction of the first signal light, the details of the second polarization separation / combination module will be described later. After being input from the 3 input / output terminal, it is output from the 4th input / output terminal. Therefore, this component is not included in the loop transmitted light output from the third input / output terminal of the first polarization splitting / combining module. Also, since the polarization crosstalk component mixed in the second signal light is a component having a polarization direction orthogonal to the polarization direction of the second signal light, this is similarly applied to the second polarization separation / combination module. After being input from the second input / output terminal, it is output from the fourth input / output terminal. Therefore, this component is not included in the loop transmitted light output from the third input / output terminal of the first polarization splitting / combining module.

  Thus, according to the optical switch of the present invention, the polarization crosstalk component does not affect the switch operation even when the third polarization-maintaining optical fiber is long. That is, it is possible to realize an optical switch that is not affected by polarization crosstalk and that is guaranteed to operate stably.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows one configuration example according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood. It is not limited to. In the following description, specific equipment and conditions may be used. However, these materials and conditions are only one of preferred examples, and are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted.

<First Embodiment>
The structure and operation of the optical switch according to the first embodiment will be described with reference to FIG.

(Construction)
This optical switch includes a first polarization separation / combination module 10, a second polarization separation / combination module 18, a first polarization plane preserving optical fiber 12, a second polarization plane preserving optical fiber 16, and a first polarization plane conversion. Unit 14, a third polarization plane-maintaining optical fiber 22, a fourth polarization plane-maintaining optical fiber 26, and a second polarization plane converter 24.

  The first polarization separation / combination module 10 includes a first input / output terminal 10-1 to which one end of an input optical fiber 32-2 for inputting signal light is coupled, and a first input / output terminal 10-1. It has a second input / output end 10-2 to which one end of the first polarization plane preserving optical fiber 12 is coupled to the opposite side, and a third input / output end 10-3 for outputting the switched signal light. .

  The second polarization separation / combination module 18 includes a first input / output end 18-1 for coupling the other end of the second polarization plane-maintaining optical fiber 16, and a third polarization on the side facing the first input / output end 18-1. A second input / output end 18-2 for coupling one end of the wavefront-maintaining optical fiber 22, a third input / output end 18-3 for coupling one end of the fourth polarization-maintaining optical fiber 26, and a third input / output end 18- A fourth input / output terminal 18-4 for outputting a polarization crosstalk component is provided on the side facing 3.

  The first polarization-maintaining optical fiber 12 has one end coupled to the second input / output terminal 10-2 of the first polarization separation / combination module 10, and the second polarization-maintaining optical fiber 16 has the second polarization separation The other end is coupled to the first input / output end 18-1 of the combining module 18, and the other end of the first polarization-maintaining optical fiber 12 and one end of the second polarization-maintaining optical fiber 16 are converted to the first polarization plane. It is connected via a portion 14 (set to a position indicated as A in FIG. 1).

  The third polarization-maintaining optical fiber 22 has one end coupled to the second input / output end 18-2 of the second polarization separation / combination module 18, and includes an optical coupler 20. One end of the fiber 26 is coupled to the third input / output end 18-3 of the second polarization separation / combination module 18, and the other end of the third polarization plane preserving optical fiber 22 and the fourth polarization plane preserving optical fiber 26 are connected. The other end is connected via a second polarization plane converter 24 (set at a position indicated by B in FIG. 1).

  As a polarization maintaining optical fiber suitable for use as the first to fourth polarization maintaining optical fibers, a PANDA type optical fiber shown in FIG. 2 is representative. This optical fiber has a polarization maintaining property by forming a stress applying portion in the vicinity of the core and applying a strong stress to the core.

FIG. 2 is a diagram showing a schematic structure of a cross section cut perpendicular to the light propagation direction of a PANDA type optical fiber, which is a polarization-maintaining optical fiber. A stress applying portion 144 is formed in a clad 140 surrounding the core 142 through which light is guided so as to sandwich the core 142. For example, the clad 140 is made of SiO ₂ , the core 142 is made of SiO ₂ doped with GeO ₂ , and the stress applying portion 144 is made of SiO ₂ doped with B ₂ O ₃ .

  By forming in this way, in FIG. 2, in the direction of the slow axis set in the plane perpendicular to the light propagation direction of the PANDA type optical fiber, and the direction of the fast axis perpendicular to the slow axis The equivalent refractive index for light guided through the core 142 is different. That is, since a stress applying portion having a refractive index higher than the refractive index of the clad 140 is disposed near the core 142, the equivalent refractive index for light in which the vibration direction of the electric field vector of light is parallel to the slow axis direction is The oscillation direction of the electric field vector of light becomes higher than the equivalent refractive index for light parallel to the fast axis direction. Due to the asymmetry of the equivalent refractive index, the polarization plane of the light input to the PANDA type optical fiber is preserved and propagated.

  In other words, in the PANDA type optical fiber, if the polarization plane of linearly polarized light is input in accordance with the slow axis (or fast axis) shown in Fig. 2, it propagates through the PANDA type optical fiber while maintaining the polarization state. Even at the emission end, it is possible to obtain only a linearly polarized light component whose polarization plane coincides with the slow axis (or fast axis).

  In the following description, for the sake of convenience, in the schematic configuration diagram of the optical switch shown in FIG. 1, the polarization direction of light propagating through the polarization-maintaining optical fiber that is an optical transmission path is defined as follows. . Polarized light whose electric field vector oscillation direction is perpendicular to the paper surface of FIG. 1 is called TE (Transverse-Electric Modes) polarized light, and the direction perpendicular to the paper surface is called the TE direction. Also, polarized light in which the vibration direction of the electric field vector of light is parallel to the paper surface is called TM (Transverse-Magnetic Modes) polarized light, and the direction parallel to the paper surface is called TM direction. Of course, the possibility of using the optical switch of the present invention is not limited to the above case.

  Also, in the following explanation, when light is incident on a polarization separation / combination module such as the first polarization separation / combination module 10, it corresponds to the vibration direction of the electric field vector with respect to the polarization plane selective reflection surface of the polarization separation / combination module of the incident light. The components to be defined are defined as follows. That is, the component that the electric field vector oscillates in the direction parallel to the incident surface of the incident light incident on the polarization plane selective reflection surface is the p component, and the component that the electric field vector oscillates in the direction perpendicular to the incident surface of the incident light is the s component. I will call it.

  For example, when light is incident on the first polarization separation / combination module 10, the electric field is parallel to the incident surface with respect to the polarization plane selective reflection surface 10R of the polarization separation / combination element constituting the first polarization separation / combination module 10 The component that the vector vibrates is the p component, and the component that the electric field vector vibrates in the direction perpendicular to the incident surface of the incident light is the s component. The same applies to the second polarization separation / combination module 18. The same applies to the third and fourth polarization splitting / combining modules used in the second embodiment and the third embodiment described later.

  As the polarization separation / combination module such as the first polarization separation / combination module 10, a suitable one can be selected from commercially available polarization beam splitters.

Further, it is desirable that the third polarization-maintaining optical fiber 22 has a large nonlinear optical effect in addition to the property of polarization plane preservation. In order to increase the nonlinear optical effect, the optical fiber core (corresponding to the core 142 shown in FIG. 2) is heavily doped with GeO ₂ to increase the nonlinear optical constant γ (W ⁻¹ km ⁻¹ ) based on the optical Kerr effect. In order to increase the optical energy density in the optical fiber, the optical field density (MFD: Mode Field Diameter) which is a waveguide mode cross-sectional area of the optical fiber is reduced.

For example, an ordinary optical fiber with an MFD of 8 μm is about γ = 2 km ⁻¹ W ⁻¹ , whereas an optical fiber with an MFD of 3.6 μm and γ = 20 km ⁻¹ W ⁻¹ is an order of magnitude larger. It is commercially available. In addition, optical fibers with high optical nonlinearity have been developed, such as a fiber having a cavity formed in a clad called a “Holey fiber” and a photonic bandgap fiber. In the future, it is naturally expected that the polarization-maintaining optical fiber will incorporate the above-described device, and that an optical fiber having high optical nonlinearity while having the property of maintaining the polarization plane will be developed.

  In the following description, the direction of the vibration plane (direction of the polarization plane) of the electric field vector of the input signal light that propagates through the input optical fiber 32-2 and is input to the first polarization splitting / combining module 10 is parallel to the plane of the paper. It is assumed that it is inclined 45 ° with respect to the surface. The incident surface of the incident light incident on the polarization plane selective reflection surface 10R of the polarization separation / combination element constituting the first polarization separation / combination module 10 is the vibration surface of the electric field vector of the incident signal light 32-2. It is assumed that it is set to be parallel to the direction.

  That is, in FIG. 1, the direction of the vibration surface of the electric field vector of the input signal light propagating through the input optical fiber 32-2 to the first polarization separation / combination module 10 is inclined by 45 ° with respect to the plane parallel to the paper surface. ing. The incident surface of the incident light incident on the polarization plane selective reflection surface 10R of the first polarization separation / combination module 10 is set to be parallel to the direction of the vibration surface of the electric field vector of the input signal light.

  One end of the first polarization-maintaining optical fiber 12 is coupled to the second input / output terminal 10-2 of the first polarization separation / combination module 10, but the optical axis of the first polarization-maintaining optical fiber 12 (here, , Slow axis) and the direction of the vibration surface of the electric field vector of the input signal light propagating through the input optical fiber 32-2 are coupled in parallel. Therefore, the polarization direction of the signal light output from the second input / output terminal 10-2 of the first polarization separation / combination module 10 and propagating through the first polarization-maintaining optical fiber 12 is the first polarization-maintaining optical fiber 12. Parallel to the direction of the slow axis. In the following description, unless otherwise specified, the optical axis of the polarization-maintaining optical fiber refers to the slow axis.

  Here, for convenience of explanation below, it is assumed that the direction of the slow axis of the first polarization plane-maintaining optical fiber 12 is inclined by 45 ° with respect to a plane parallel to the paper surface, and the second polarization plane is preserved. The direction of the slow axis of the optical fiber 14 is assumed to be set parallel to the paper surface. In this way, the slow axis of the first polarization-maintaining optical fiber 12 and the slow axis of the second polarization-maintaining optical fiber 14 can be set to be inclined by 45 ° with respect to each other, as will be described later. This is because the other end of 12 and one end of the second polarization-maintaining optical fiber 16 are connected via the first polarization plane converter 14.

  Of course, unlike the above, the direction of the polarization plane of the input signal light can be set as the direction with respect to the fast axis to configure the optical switch. Description is omitted.

  In the first polarization separation / combination module 10, the p-polarization component input from the first input / output terminal 10-1 is output to the third input / output terminal 10-2, and from the second input / output terminal 10-2. The input s-polarized component is output to the third input / output terminal 10-3. The p-polarized component input from the second input / output terminal 10-2 is output to the first input / output terminal 10-1.

Further, the length of the path from the second input / output terminal 10-2 of the first polarization separation / combination module 10 to the first polarization plane converter 14, that is, the length of the first polarization plane preserving optical fiber 12, is denoted by l ₁ ( also referred to as path L _1.), the length of the path from the first polarization plane conversion portion 14 to the first output terminal 18-1 of the second polarization splitting-combining module 18, i.e. the second polarization maintaining optical fiber The length of 16 is l ₂ (also referred to as path L ₂ ), the path from the second input / output end 18-2 of the second polarization splitting / combining module 18 to the second polarization plane converter 24, that is, the third The length of the polarization-maintaining optical fiber 22 is l ₃ (also referred to as a path L ₃ ), and extends from the second polarization plane converter 24 to the third input / output terminal 18-3 of the second polarization separation / combination module 18. The path, that is, the length of the fourth polarization-maintaining optical fiber 26 is assumed to be l ₄ (also referred to as path L ₄ ).

  The first polarization plane conversion unit 14 and the second polarization plane conversion unit 24 can be configured by using a Faraday rotator, and can also be realized by the structure described with reference to FIG. FIG. 3 is a diagram for explaining the structure of the polarization plane conversion unit. The optical fiber 170 and the optical fiber 172 are PANDA type optical fibers. In FIG. 3, sections 174 and 176 show a schematic structure of a section of PANDA type optical fibers 170 and 176 cut perpendicular to the light propagation direction. This cross-sectional structure is the same as the PANDA type optical fiber described above with reference to FIG.

  As shown in the upper part of FIG. 3, the cut surfaces of the PANDA type optical fibers 170 and 172 are arranged so that their slow axes are orthogonal to each other. The first polarization plane conversion unit 14 and the second polarization plane conversion unit 24 cut one end of the PANDA type optical fiber perpendicularly to the light propagation direction, and then each slow axis (fast axis) is orthogonal to each other. It is formed by adhering to the core so that the core matches and fusing.

(Operation)
With reference to FIG. 1, the operation principle of the optical switch of the first embodiment will be described. According to the optical switch of the present invention, linearly polarized signal light that is input signal light to the optical switch is input to the first input / output terminal 10-1 of the first polarization separation / combination module 10, and the signal light Are stored and output from the second input / output terminal 10-2 of the first polarization separation / combination module 10.

  At this time, the direction of the vibration plane of the electric field vector of the input signal light propagating through the input optical fiber 32-2 coupled to the first input / output terminal 10-1 of the first polarization separation / combination module 10 is It is inclined 45 ° with respect to the parallel plane. Note that the incident plane of incident light that enters the polarization plane selective reflection surface 10R of the first polarization separation / combination module 10 is set to be parallel to the direction of the vibration plane of the electric field vector of the input signal light. I want to be. That is, the direction of the polarization plane of the signal light output from the second input / output terminal 10-2 of the first polarization separation / combination module 10 matches the direction of the optical axis of the first polarization plane preserving optical fiber 12. Is set to

  In order to operate in this way, linearly polarized signal light that is input signal light to the optical switch is input to the first input / output terminal 10-1 of the first polarization separation / combination module 10 as p-polarized light. In general, the polarization plane selective reflection surface of the polarization beam splitting / combining element has a property of transmitting only the p component and reflecting only the s component. Therefore, the signal light input to the first input / output terminal 10-1 of the first polarization separation / combination module 10 as p-polarized light remains the p-polarized light, and the first input / output of the first polarization separation / combination module 10 remains unchanged. The signal is output from the second input / output terminal 10-2 installed on the side facing the terminal 10-1.

  The signal light output from the second input / output terminal 10-2 of the first polarization separation / combination module 10 is sent from one end of the first polarization plane preserving optical fiber 12 to the optical axis of the first polarization plane preserving optical fiber 12. The polarization plane is input so as to match and propagates, reaches the first polarization plane conversion unit 14 installed at the other end of the first polarization plane preserving optical fiber 12, passes through it, and then passes through the second polarization plane. Input to one end of the storage optical fiber 16.

  As described above, the first polarization plane conversion unit 14 is fused so that the optical axis of the first polarization plane preserving optical fiber 12 and the optical axis of the second polarization plane preserving optical fiber 16 form an angle of 45 ° with each other. It is formed by doing. Therefore, the signal light that has passed through the first polarization plane conversion unit 14 includes an electric field vector component (TM component) parallel to the optical axis of the second polarization plane preserving optical fiber 16 and an electric field vector component (TE component) perpendicular to the optical axis. Is propagated through the second polarization-maintaining optical fiber 16 and input to the first input / output terminal 18-1 of the second polarization separation / combination module 18.

  The signal light input to the second polarization separation / combination module 18 via the first input / output terminal 18-1 of the second polarization separation / combination module 18 is the same as the first signal light whose polarization planes are orthogonal to each other. The signals are separated into two signal lights and output from the second input / output terminal 18-2 and the third input / output terminal 18-3 of the second polarization separation / combination module 18, respectively.

Referring to FIGS. 4A to 4C, the first polarization plane converter 14 connecting the other end of the first polarization-maintaining optical fiber 12 and one end of the second polarization-maintaining optical fiber 16 is connected. The function of will be described. 4 (A) to 4 (C), a path L ₁ constituted by the first polarization-preserving optical fiber 12 on the left side of the drawing with respect to the point A where the first polarization plane conversion unit 14 is installed. The right side corresponds to the path L ₂ constituted by the second polarization-maintaining optical fiber 16.

  The circular diagrams depicted on the left and right sides of FIGS. 4 (A) to 4 (C) are cut along a plane perpendicular to the optical axis of the first polarization plane converter 14 and the second polarization plane preserving optical fiber 16, respectively. FIG. In each of the cross-sectional views, the black circle at the center indicates the core, and the two circular portions drawn side by side with shadow are the stress applying portions. Further, in each of the cross-sectional views, the black thick arrow and the white arrow indicate the polarization component oscillation direction of the signal light propagating through the first polarization plane preserving optical fiber 12 and the second polarization plane preserving optical fiber 16, respectively. It is a vector display which shows direction and its intensity | strength.

As described above, the optical axis of the first polarization-maintaining optical fiber 12 is disposed at an angle of 45 ° with the paper surface, and the incident surface of the signal light with respect to the polarization plane selective reflection surface 10R of the first polarization separation / combination module 10 Is set to be parallel. Therefore, the vector display indicating the polarization plane of the signal light propagating through the first polarization plane preserving optical fiber 12 (path L ₁ ) toward the first polarization plane converter 14 is a direction parallel to the optical axis as shown in the figure. Facing. As described above, the first polarization plane conversion unit 14 installed at the position indicated by the point A is fused so as to form an angle of 45 ° with the optical axis of the second polarization plane preserving optical fiber 16. Therefore, the polarization plane of the signal light that has passed through the first polarization plane converter 14 has polarization components in the slow axis and fast axis directions, as shown in the drawing on the right side of FIG. Propagates through the second polarization-maintaining optical fiber 16.

  In FIG. 1, the slow axis of the second polarization-maintaining optical fiber 16 is depicted as being set in a direction parallel to the paper surface. Further, the fast axis of the second polarization-maintaining optical fiber 16 is drawn as being set in a direction perpendicular to the paper surface. Therefore, the electric field vector component parallel to the slow axis of the signal light propagating through the second polarization plane preserving optical fiber 16 is TM polarization, and the electric field vector component parallel to the fast axis of the signal light is TE polarization.

The first signal light is output from the second input / output terminal 18-2 of the second polarization separation / combination module 18, and the third polarization-maintaining optical fiber 22 (path L ₃ ) and the fourth polarization-maintaining optical fiber 26 are output. It propagates clockwise (sometimes referred to as the CW direction) through an optical fiber loop composed of (path L ₄ ). That is, the second polarization plane conversion provided at the other end of the third polarization-maintaining optical fiber 22 is input from one end of the third polarization-maintaining optical fiber 22 and propagates through the third polarization-maintaining optical fiber 22. The signal passes through the unit 24 and is input to the fourth polarization-maintaining optical fiber 26.

  The second polarization plane conversion unit 24 is formed by, for example, rotating the optical axis of the third polarization plane preserving optical fiber 22 and the optical axis of the fourth polarization plane preserving optical fiber 26 by being rotated by 90 ° with respect to each other. ing. Therefore, the polarization plane of the first signal light propagated through the third polarization-maintaining optical fiber 22 is rotated by 90 ° and input to the fourth polarization-maintaining optical fiber 26. The first signal light propagated through the fourth polarization-maintaining optical fiber 26 is input to the third input / output terminal 18-3 of the second polarization separation / combination module 18.

On the other hand, the second signal light is output from the third input / output terminal 18-3 of the second polarization separation / combination module 18, and the fourth polarization plane preserving optical fiber 26 (path L ₄ ) and the third polarization plane preserving light are output. It propagates counterclockwise (sometimes referred to as CCW direction) through an optical fiber loop composed of the fiber 22 (path L ₃ ). That is, the second polarization plane conversion provided at the other end of the fourth polarization plane preserving optical fiber 26 is input from one end of the fourth polarization plane preserving optical fiber 26 and propagates through the fourth polarization plane preserving optical fiber 26. The signal passes through the unit 24 and is input to the third polarization-maintaining optical fiber 22. As in the case of the first signal light, the polarization plane of the second signal light propagated through the fourth polarization plane preserving optical fiber 26 is rotated by 90 ° in the second polarization plane conversion unit 24 and the third polarization plane preserving optical fiber is rotated. Entered in 22. The second signal light propagated through the third polarization-maintaining optical fiber 22 is input to the second input / output terminal 18-2 of the second polarization separation / combination module 18.

  Here, the direction of the polarization plane of the second signal light input to the second input / output end 18-2 of the second polarization separation / combination module 18 and the first input / output end 18 of the second polarization separation / combination module 18 Of the first signal light output to the second input / output terminal 18-2 of the second polarization separation / combination module 18 out of the signal light input to the second polarization separation / combination module 18 via -1 The direction is the same. Further, the direction of the polarization plane of the first signal light input to the third input / output terminal 18-3 of the second polarization separation / combination module 18 and the first input / output terminal 18- of the second polarization separation / combination module 18 Of the signal light input to the second polarization separation / combination module 18 via 1, the polarization plane of the second signal light output to the third input / output terminal 18-3 of the second polarization separation / combination module 18 The direction matches.

  Therefore, after the second signal light and the first signal light combined in the second polarization separation / combination module 18 are combined, the signal light is input to the second polarization separation / combination module 18. And propagates from the second polarization-maintaining optical fiber 16 toward the first polarization-maintaining optical fiber 12, which is in the same polarization state as when the signal light is input. The signal is input from the second input / output terminal 10-2 of the first polarization separation / combination module 10 and output from the first input / output terminal 10-1 on the side opposite to the second input / output terminal 10-2.

  That is, the signal light of linearly polarized light (the direction of the polarization plane forms an angle of 45 ° with the paper surface) input to the first input / output terminal 10-1 of the first polarization separation / combination module 10 is as described above. The first polarization separation / combination module 10 outputs the same polarization state as that inputted from the first input / output terminal 10-1. In other words, since the signal light input to this optical switch is reflected and can be regarded as being output from the first input / output terminal 10-1 of the first polarization splitting / combining module 10, it is referred to as the first in this way. The signal light output from the first input / output terminal 10-1 of the polarization splitting / combining module 10 may be referred to as loop reflected light.

Next, consider a case where control light is input from the optical coupler 20 provided with the third polarization-maintaining optical fiber 22. In this case, the phase velocity of the first signal light propagating through the third polarization-maintaining optical fiber 22 changes. When the length (= l ₃ ) of the third polarization-maintaining optical fiber 22 (path L ₃ ) is adjusted according to the intensity of the control light, and the first signal light propagates through the third polarization-maintaining optical fiber 22 The phase difference φ given by the above equation (1) can be set exactly equal to π by comparing the case where the control light is present with and without the control light.

Since the nonlinear optical constant γ (W ^-1 km ^-1 ) and control light power P (W) are determined by the basic configuration of the optical communication system in which this optical switch is used, an optical fiber loop is formed. By adjusting the length of the optical fiber, that is, L (km) which is the length (= l ₃ ) of the third polarization-maintaining optical fiber 22 (path L ₃ ), it is clear from the equation (1) As can be seen, the phase difference φ given by equation (1) can be set exactly equal to π.

  In the second polarization separation / combination module 18, after the second signal light and the first signal light are combined, they propagate from the second polarization-maintaining optical fiber 16 toward the first polarization-maintaining optical fiber 12. To do. Then, after passing through the first polarization plane converter 14, the first polarization plane preserving optical fiber 12 is propagated as output signal light having a polarization plane orthogonal to the direction of the polarization plane of the input signal light. Here, the input signal light refers to signal light that propagates from the first polarization plane-maintaining optical fiber 12 through the first polarization plane conversion unit 14 and propagates toward the second polarization plane-maintaining optical fiber 16. . The output signal light refers to signal light that propagates from the second polarization-maintaining optical fiber 16 through the first polarization-plane converting unit 14 and propagates toward the first polarization-maintaining optical fiber 12.

  Therefore, this output signal light is input from the second input / output terminal 10-2 of the first polarization splitting / combining module 10 and output from the third input / output terminal 10-3. Hereinafter, the signal light output from the third input / output terminal 10-3 of the first polarization separation / combination module 10 may be referred to as loop transmitted light.

Here, the effective refractive index of the slow axis of the first to fourth polarization-maintaining optical fibers PANDA type optical fiber is n _s , the effective refractive index of the fast axis is n _f, and the direction of the slow axis is TM polarized It is assumed that the fast axis direction matches the polarization plane of the TE polarization.

  First, starting from the first polarization plane converter 14 installed at the point A, the second polarization plane preserving optical fiber 16 is propagated as a TM polarization component, and the first input of the second polarization separation / combination module 18 is transmitted. Consider the propagation path of the signal light component input to the output terminal 18-1. This signal light component propagates through the second polarization-maintaining optical fiber 16 as the TM polarization component as the first signal light, and is p-polarized at the first input / output terminal 18-1 of the second polarization separation / combination module 18. It is input as wave light, is output from the second input / output terminal 18-2, and propagates through the third polarization-maintaining optical fiber 22 as TM polarization component. Then, by passing through the second polarization plane conversion unit 24, it propagates through the fourth polarization plane preserving optical fiber 26 as a TE component, and is transmitted to the third input / output end 18-3 of the second polarization separation / combination module 18 It is input as polarized light, output from the first input / output terminal 18-1, propagates through the second polarization-maintaining optical fiber 16 as a TE polarization component, and enters the first polarization plane converter 14 installed at the point A. Return.

Propagating the second polarization plane preserving optical fiber 16 as a TM polarization component, the optical path length of the propagation path of the signal light component input to the first input / output end 18-1 of the second polarization separation / combination module 18 is The signal light components are added in the order of propagation paths and given by the following equation (2).
n _s l ₂ + n _s l ₃ + n _f l ₄ + n _f l ₂ (2)
Here, l ₂ , l ₃ , and l ₄ are the lengths of the paths L ₂ , L ₃ , and L ₄ , respectively.

That is, since the path L ₂ formed by the second polarization-maintaining optical fiber 16 propagates as TM polarization, the optical path length is n _s l ₂ and the path L formed by the third polarization-maintaining optical fiber 22 _{Since 3} also propagates as TM polarization, the optical path length is n _s l ₃ . Further, by passing through the second polarization plane conversion portion 24, the optical path length because the propagation path L ₄ formed by the fourth polarization maintaining optical fiber 26 as TE component is n _f l _4. Further, since the path L ₂ formed by the second polarization-maintaining optical fiber 16 propagates as the TE polarization component, the optical path length is n _f l ₂ . That is, the total optical path length is given by the above equation (2).

On the other hand, the optical path length of the propagation path of the signal light component propagated through the second polarization-maintaining optical fiber 16 as the TE polarization component and input to the first input / output terminal 18-1 of the second polarization separation / combination module 18 Is added in the order of the propagation path of the signal light component, and is given by the following equation (3).
n _f l ₂ + n _f l ₄ + n _s l ₃ + n _s l ₂ (3)
Here, l ₂ , l ₃ , and l ₄ are the lengths of the paths L ₂ , L ₃ , and L ₄ , respectively.

That is, since the path L ₂ formed by the second polarization-maintaining optical fiber 16 propagates as TE polarized light, the optical path length is n _f l ₂ and the path L formed by the fourth polarization-maintaining optical fiber 26 ₄ optical path length so propagated as TE polarization also is n _f l _4. Further, since it passes through the second polarization plane conversion unit 24 and propagates through the path L ₃ formed by the third polarization plane preserving optical fiber 22 as TM polarization, the optical path length is n _s l ₃ . Further, since the path L ₂ formed by the second polarization-maintaining optical fiber 16 is propagated as TM polarization, the optical path length is n _s l ₂ . That is, the total optical path length is given by the above equation (3).

  The control light follows a path that propagates in the CW direction, which is the same path as the first signal light.

  Comparing the above formulas (2) and (3), the first term, the second term, the third term, the fourth term in the formula (2) and the fourth term, the third term, the second term, the first term in the formula (3) and Are equal to each other. That is, it can be seen that the first signal light and the second signal light propagate through the same optical path length.

  As described above, the optical switch of the present invention has the same path for the first signal light and the second signal light, so even if the wavelength of the signal light or the ambient temperature of the optical switch changes, its operating characteristics Thus, an optical switch in which stable operation is guaranteed can be realized.

  Starting from the first polarization plane converter 14 installed at the point A, it propagates through the second polarization-maintaining optical fiber 16 and is input to the first input / output terminal 18-1 of the second polarization separation / combination module 18. Both the TM polarization component and the TE polarization component signal light component are combined in phase in the second polarization separation / combination module 18 when no control light is input. As a result, the signal light input from the input optical fiber 32-2 to the optical switch is output as loop reflected light from the same input optical fiber 32-2 as that during input.

On the other hand, when the control light is input from the optical coupler 20, the optical Kerr effect appears in the third polarization-maintaining optical fiber 22, and the refractive index changes. Therefore, the second signal light propagating in the first signal light and the CCW direction propagating in the CW direction, each of the configured optical fiber loop from the path L ₃ to L ₄ propagate respectively, again the second polarization When both are combined by the separation / synthesis module 18, the phases of the two are shifted. As described above, the length of the third polarization-maintaining optical fiber 22 can be adjusted so that the phase shift amount φ is equal to π.

  The operation of the optical switch of the present invention will be specifically described with reference to FIG. 1, taking as an example the case of switching a time division multiplexed optical pulse signal. In FIG. 1, a schematic time waveform of transmitted light and reflected light as a result of switching of signal light, which is an optical pulse signal, control light, and optical pulse signal, is shown with the horizontal axis as a time axis. Assume a binary digital signal corresponding to “0” or “1” corresponding to the presence or absence of an optical pulse. Consider a case in which signal light in which optical pulses of wavelength λs are regularly arranged is controlled (switched) with control light comprising an optical pulse train of wavelength λp representing [1 0 1 1].

  The signal light is input to the input optical fiber 32-1 and propagates through the input optical fiber 32-2 via the optical circulator 30 to the first input / output terminal 10-1 of the first polarization separation / combination module 10. Entered. As described above, the signal light passes through the first polarization separation / combination module 10 and propagates through the first polarization-maintaining optical fiber 12, and a part of the component light is provided in the third polarization-maintaining optical fiber 22. Reach coupler 20.

  The first optical pulse that is the first optical pulse of the signal light (in FIG. 1, the optical pulse that is positioned at the rightmost position on the time axis) passes through the optical coupler 20 and is exactly the first optical pulse that is the first optical pulse of the control light. It is assumed that one optical pulse (in FIG. 1, the optical pulse located on the rightmost side on the time axis) is input from the optical coupler 20. Of course, the control light is also input to the first optical fiber through the optical coupler 20 as TM polarization.

  The optical pulse (wavelength λs) of the first signal light and the optical pulse of control light (wavelength λp) propagate through the third polarization-maintaining optical fiber 22 and the fourth polarization-maintaining optical fiber 26 while running in parallel. become. Therefore, the effective refractive index of the third polarization plane-maintaining optical fiber 22 and the fourth polarization plane-maintaining optical fiber 26 with respect to the optical pulse of the first signal light changes due to the optical Kerr effect that is generated by the optical pulse of the control light. In other words, the optical pulse of the first signal light and the optical pulse of the control light travel along the third polarization-maintaining optical fiber 22 and the fourth polarization-maintaining optical fiber 26, thereby causing the control light optical pulse. Thus, the optical pulse of the first signal light propagates through the optical path whose effective refractive index is constantly changing. On the other hand, the optical pulse (wavelength λs) of the second signal light propagates through the third polarization-maintaining optical fiber 22 in the opposite direction to the optical pulse of the first signal light without being affected by the optical pulse of the control light. Become.

As a result, the effective refractive indexes n _s and n _f appearing in the above equation (2) are different from the effective refractive indexes n _s and n _f appearing in the above equation (3). That is, the effective refractive index values appearing in the above equation (2) are n _s ′ and n _f ′.

Therefore, the lengths of the path L ₃ (third polarization-maintaining optical fiber) and the path L ₄ (fourth polarization-maintaining optical fiber) are
(n _s l ₂ + n _s 'l ₃ + n _f ' l ₄ + n _f l ₂ ) − (n _f l ₂ + n _f l ₄ + n _s l ₃ + n _s l ₂ ) = λs / 2
If the adjustment is performed, the phase difference when the optical pulse of the first signal light and the optical pulse of the second signal light are combined in the second polarization separation / combination module 18 can be set to π.

  Strictly speaking, since the wavelengths of the control light and the signal light are λp and λs, respectively, the wavelengths are different. Therefore, the group delay time difference between the control light and the signal light due to the group velocity dispersion generated in the third polarization-maintaining optical fiber 22 and the fourth polarization-maintaining optical fiber 26 is on the time axis of the optical pulse of the signal light. It must be shorter than the interval of appearance, that is, the time interval occupied by one bit of signal light (one optical pulse). However, this condition can be easily satisfied with almost no difference between the wavelength λp of the control light and the wavelength λs of the signal light. The difference between the wavelength λp of the control light and the wavelength λs of the signal light is that, as will be described later, in order to output only the switched signal light from the optical switch, an optical bandpass filter 28 set on the output side of the optical switch. Therefore, it is sufficient that the control light can be blocked and the signal light can be transmitted.

Next, when the second optical pulse next to the first optical pulse of the first signal light passes through the optical coupler 20, there is a second optical pulse that is exactly the next optical pulse of the control light. When the third optical pulse next to the second optical pulse of the signal light passes through the optical coupler 20, there is no third optical pulse that is the optical pulse next to the second optical pulse of the control light. In this case, the third optical pulse of the first signal light is in a state where the optical pulse of the control light does not exist, that is, the optical pulse of the first signal light does not travel in parallel with the optical pulse of the control light. It propagates through the optical path composed of L ₃ and L ₄ .

  Therefore, in the second polarization separation / combination module 18, the optical pulse of the first signal light and the optical pulse of the second signal light are combined in the same phase. Therefore, as will be described later, the optical pulse of the signal light is output as a loop reflected light to the first port 10-1 of the first polarization separation / combination module 10, and propagates through the input optical fiber 32-2 to the optical circulator 30. Then, the signal light is output toward the reflected light output optical fiber 36 different from the transmission path through which the signal light has propagated.

  As a result, the optical pulse train constituting the transmitted light output from the bandpass filter 28 to the transmitted light output optical fiber 29 reflects the optical pulse train constituting the control light as shown in FIG. In addition, the reflected light propagates through the optical fiber 36 for reflected light output via the optical circulator 30 as loop reflected light only when the signal light light pulse exists in the time zone when the control light light pulse does not exist. 1 is output to the outside, resulting in a pulse train as shown in FIG.

  Further, since the control light having the wavelength λp is also output from the third input / output terminal 10-3 of the first polarization separation / combination module 10, in order to extract only the switched signal light having the wavelength λs to the outside, 1 One end of the output optical fiber 27 is connected to the third input / output terminal 10-3 of the polarization splitting / combining module 10, and the other end of the output optical fiber 27 is set to have a transmission wavelength center of λs and It is necessary to connect an optical bandpass filter 28 having a characteristic capable of blocking the wavelength λp.

  The optical pulse of the first signal light output to the first input / output terminal 10-1 of the first polarization splitting / combining module 10 as loop reflected light is transmitted through the transmission path unless the optical circulator 30 is arranged. The process proceeds in reverse, and is sent back to the transmitting side. In general, in time-multiplexed optical communication, it is not preferable that a part of the transmission signal is transmitted backward from the reception side to the transmission side. Therefore, the first polarization separation is performed as the loop reflected light using the optical circulator 30. It is desirable that the optical pulse of the signal light output to the first input / output terminal 10-1 of the combining module 10 is output to a transmission path different from the transmission path through which the signal light has propagated.

  Here, in the second polarization separation / combination module 18, after the second signal light and the first signal light are combined, the second polarization plane preserving optical fiber 16 is directed to the first polarization plane preserving optical fiber 12. The reason for propagating through the first polarization plane preserving optical fiber 12 as output signal light having a polarization plane orthogonal to the direction of the polarization plane of the input signal light is transmitted through the first polarization plane conversion unit 14 This will be described with reference to FIGS. 4 (A) to 4 (C).

  First, a case where the optical pulse of the control light is not input to the third polarization-maintaining optical fiber 22 through the optical coupler 20 in synchronization with the time when the optical pulse of the signal light passes through the optical coupler 20 will be considered. In this case, the signal light propagating through the third polarization-maintaining optical fiber 22 is not subjected to phase intermodulation based on the optical Kerr effect.

  Therefore, the first signal light and the second signal light are combined by the second polarization separation / combination module 18 and output from the first input / output terminal 18-1 of the second polarization separation / combination module 18, The TM and TE components of the signal light that travels through the two polarization-maintaining optical fibers 16 toward the first polarization plane converter 14 are as shown in FIG. 4 (B). That is, as shown in FIG. 4 (A), the first input / output of the second polarization separation / combination module 18 is transmitted through the first polarization plane converter 14 and propagated through the second polarization plane preserving optical fiber 16. This is the same as the TM and TE components of the signal light when input to the end 18-1.

  On the other hand, a case is considered in which the optical pulse of the control light is also input to the third polarization-maintaining optical fiber 22 through the optical coupler 20 in synchronization with the time when the optical pulse of the signal light passes through the optical coupler 20. In this case, the signal light propagating through the third polarization-maintaining optical fiber 22 is subjected to phase intermodulation based on the optical Kerr effect, and the optical pulse of the first signal light and the optical pulse of the second signal light are second polarized. The phase difference when combined in the wave separation / combination module 18 is π.

In this case, the phase of the optical pulse of the first signal light reaches the polarization plane selective reflection surface 18R of the second polarization separation / combination module 18 with a phase delay of π compared to the phase of the optical pulse of the second signal light. To do. It propagates from the third polarization-maintaining optical fiber 22 (path L ₃ ) to the fourth polarization-maintaining optical fiber 26 (path L ₄ ) and inputs to the third input / output terminal 18-3 of the second polarization separation / combination module 18 Since the polarization plane of the first signal light is an s-polarization component with respect to the polarization plane selective reflection surface 18R, it is reflected by the polarization plane selection reflection surface 18R and the second polarization plane preserving optical fiber 16 (path L) ₂ ) is propagated as a TE polarization component and reaches the first polarization plane converter 14. Therefore, the TE polarization component of the signal light in the first polarization plane converter 14 of the second polarization plane preserving optical fiber 16 (path L ₂ ) is as shown in FIG. The second polarization splitting / synthesizing module is subjected to phase intermodulation based on the optical Kerr effect by the optical pulse of the control light, and the phase of the optical pulse of the first signal light is delayed by π compared to the optical pulse of the second signal light. Since the 18 polarization plane selective reflection surface 18R has been reached, this delay is reflected in the first polarization plane conversion unit 14 of the second polarization plane preserving optical fiber 16, where the TE polarization component is shown in FIG. ) And the TE polarization component shown in (B), the direction is 180 ° (π in terms of phase).

The signal light in the polarization state shown in FIG. 4 (C) passes through the first polarization plane converting unit 14 from the second polarization plane preserving optical fiber 16 (path L ₂ ) and passes through the first polarization plane preserving optical fiber 12 (path L). ₁ ) will be propagated. In other words, the polarization direction of the signal light propagating through the first polarization-maintaining optical fiber 12 (path L ₁ ) is a vector addition of the TM component and the TE component in the second polarization-maintaining optical fiber 16 (path L ₂ ). Therefore, as shown on the left side of FIG. 4 (C), it has a plane of polarization orthogonal to the polarization direction of the input signal light (the polarization direction shown in FIGS. 4 (A) and 4 (B)). become.

  The polarization direction of the input signal light is input as the p-polarization component to the polarization selective reflection surface 10R of the first polarization separation / combination module 10, whereas the output signal light is The direction of the polarization plane is output from the third input / output terminal 10-3 of the first polarization separation / combination module 10 as the s polarization component with respect to the polarization selective reflection surface 10R of the first polarization separation / combination module 10 Will be.

  Therefore, in this case, the light is reflected by the polarization selective reflection surface 10R of the first polarization separation / combination module 10 and output from the third input / output terminal 10-3 of the first polarization separation / combination module 10, and the output optical fiber. Only the signal light is selected by the band pass filter 28 via 27, and is output to the outside as transmitted light from the transmitted light output optical fiber 29.

  Next, polarization crosstalk is examined. In this optical switch, the polarization crosstalk mainly occurs in the third polarization-maintaining optical fiber 22 having the longest fiber length. Since the polarization crosstalk component mixed in the first signal light is a component having a polarization direction orthogonal to the polarization direction of the first signal light, the third input / output terminal 18 of the second polarization separation / combination module 18 is used. After being input from -3, it is output from the fourth input / output terminal 18-4. That is, the polarization crosstalk component mixed into the first signal light is a p-polarization component with respect to the polarization plane selective reflection surface 18R of the second polarization separation / combination module 18. On the other hand, the polarization direction of the first signal light is an s-polarization component with respect to the polarization plane selective reflection surface 18R.

  That is, the first signal light not including the polarization crosstalk component is input from the third input / output terminal 18-3 and then output to the first input / output terminal 18-1. Therefore, the polarization crosstalk component mixed in the first signal light cannot reach the first polarization plane conversion unit 14 installed at the point A and does not interfere with the signal light. That is, when loop transmitted light is output from the third input / output terminal 10-3 of the first polarization separation / combination module 10, a part of the signal light component is output as a noise component from the first input / output terminal 10-1. It will not be done.

  The polarization crosstalk component mixed in the second signal light is the polarization direction (polarization plane selective reflection) orthogonal to the polarization direction of the second signal light (p polarization component relative to the polarization plane selective reflection surface 18R). This is also a component having an s-polarization component) with respect to the surface 18R, and this is similarly input from the second input / output end 18-2 of the second polarization separation / combination module 18 and then the fourth input / output end Output from 18-4.

  That is, the second signal light not including the polarization crosstalk component is input from the second input / output terminal 18-2 and then output to the first input / output terminal 18-1. For this reason, the polarization crosstalk component mixed in the second signal light cannot reach the first polarization plane conversion unit 14 installed at the point A, and does not interfere with the signal light. That is, when loop transmitted light is output from the third input / output terminal 10-3 of the first polarization separation / combination module 10, a part of the signal light component is output as a noise component from the first input / output terminal 10-1. It will not be done.

  Thus, according to the optical switch of the present invention, the polarization crosstalk component does not affect the switch operation even when the third polarization-maintaining optical fiber is long. That is, it is possible to realize an optical switch that is not affected by polarization crosstalk and that is guaranteed to operate stably.

  By adding an optical nonlinear controller described below to the optical switch of the present invention described above, under the condition that the type of optical fiber used for the optical fiber loop and the intensity of the control light are determined as design parameters, The length of the optical fiber that exhibits the optical Kerr effect can be substantially shortened, and a compact optical switch can be provided. In general, when an optical switch is used in time-multiplexed optical communication, the optical fiber material that can be used or the intensity of control light may be determined in advance as a design precondition depending on various conditions such as the wavelength of the optical carrier wave and its intensity. Many.

<Second Embodiment>
The structure and operation of the optical switch according to the second embodiment will be described with reference to FIG.

(Construction)
This optical switch includes a first polarization separation / combination module 10, a second polarization separation / combination module 18, a first polarization plane preserving optical fiber 12, a second polarization plane preserving optical fiber 16, and a first polarization plane conversion. Unit 14, third polarization-maintaining optical fiber 22, sixth polarization-maintaining optical fiber 60, seventh polarization-maintaining optical fiber 64, second polarization-plane converting unit 62, and optical nonlinear controller 50. It is. In addition, it is preferable to include an optical bandpass filter 28 and an optical circulator 30.

  The first polarization separation / combination module 10 has a first input / output end 10-1 for inputting signal light and a side opposite to the first input / output end 10-1 for coupling one end of the first polarization plane preserving optical fiber 12. The second input / output terminal 10-2 and the third input / output terminal 10-3 for outputting the switched signal light.

  The second polarization separation / combination module 18 includes a first input / output end 18-1 for coupling the other end of the second polarization plane-maintaining optical fiber 16, and a third polarization on the side facing the first input / output end 18-1. A second input / output end 18-2 for coupling one end of the wavefront-maintaining optical fiber 22, a third input / output end 18-3 for coupling one end of the seventh polarization-maintaining optical fiber 64, and a third input / output end 18- A fourth input / output terminal 18-4 for outputting a polarization crosstalk component is provided on the side facing 3.

  The first polarization-maintaining optical fiber 12 has one end coupled to the second input / output terminal 10-2 of the first polarization separation / combination module 10, and the second polarization-maintaining optical fiber 16 has the second polarization separation The other end is coupled to the first input / output end 18-1 of the combining module 18, and the other end of the first polarization-maintaining optical fiber 12 and one end of the second polarization-maintaining optical fiber 16 are converted to the first polarization plane. They are connected via the part 14.

  The third polarization plane preserving optical fiber 22 has one end coupled to the second input / output end 18-2 of the second polarization separation / combination module 18, and includes an optical coupler 20. One end of the fiber 64 is coupled to the third input / output end 18-3 of the second polarization separation / combination module 18, and the other end of the seventh polarization plane preserving optical fiber 64 and the sixth polarization plane preserving optical fiber 60 are connected. One end is connected via the second polarization plane converter 62.

  The optical nonlinear controller 50 includes a third polarization separation / combination module 52, a fourth polarization plane preserving optical fiber 54, a fifth polarization plane preserving optical fiber 58, and a third polarization plane converter 56. ing. The other end of the third polarization-maintaining optical fiber 22 is coupled to the first input / output terminal 52-1 of the third polarization separation / combination module 52. One end of the fourth polarization-maintaining optical fiber 54 is coupled to the second input / output end 52-2 on the side facing the first input / output end 52-1. The other end of the fifth polarization-maintaining optical fiber 58 is coupled to the third input / output end 52-3. The other end of the sixth polarization-maintaining optical fiber 60 is coupled to the fourth input / output end 52-4 on the side facing the third input / output end 52-3.

  Further, the other end of the fourth polarization-maintaining optical fiber 54 and one end of the fifth polarization-maintaining optical fiber 58 are connected via a third polarization-plane conversion unit 56 set at the position of point C shown in FIG. Has been. The third polarization plane converter 56 is formed by, for example, rotating the optical axis of the fourth polarization plane preserving optical fiber 54 and the optical axis of the fifth polarization plane preserving optical fiber 58 by being rotated by 90 ° with respect to each other. ing.

  The optical switch of the second embodiment shown in FIG. 5 has the same structure as that of the optical switch of the first embodiment shown in FIG. 1 except that the optical nonlinear controller 50 is provided. The function of this part is common to both. Therefore, the description of the operation based on the structure of the common part and the function thereof will be omitted, and the effects that are manifested based on the structure of the optical nonlinear controller 50 and the function thereof will be described below.

The length of the path from the second input / output end 52-2 of the third polarization separation / combination module 52 to the third polarization plane converter 56, that is, the length of the fourth polarization plane preserving optical fiber 54 is expressed as l ₄ (path L ₄ )), the length of the path from the third input / output end 52-3 of the third polarization separation / combination module 52 to the third polarization plane converter 56, that is, the fifth polarization plane preserving optical fiber 58 Let the length be l ₅ (also referred to as path L ₅ ).

(Operation)
According to the optical switch of the second embodiment, the first signal light propagated in the CW direction through the third polarization-maintaining optical fiber 22 is the first polarization separation / combination module 52 of the optical nonlinear controller 50. The second signal light that is input to the input / output terminal 52-1 and propagates in the CCW direction through the sixth polarization-maintaining optical fiber 60 is the fourth input / output of the third polarization separation / combination module 52 of the optical nonlinear controller 50. Input to end 52-4.

  The first signal light is input from the first input / output terminal 52-1 of the third polarization separation / combination module 52 and output from the second input / output terminal 52-2 on the side facing the first input / output terminal 52-1. Is input to the fourth polarization-maintaining optical fiber 54, propagates through the fourth polarization-maintaining optical fiber 54, and the third polarization-plane converting unit 56 rotates the polarization plane by 90 °, so that the fifth polarization-maintaining optical fiber 58 is input to the third input / output terminal 52-3 of the third polarization separation / combination module 52. Then, it is output from the second input / output terminal 52-2 of the third polarization separation / combination module 52 and propagates again through the fourth polarization plane preserving optical fiber 54, and the polarization plane is again 90 ° at the third polarization plane converter 56. Rotated, propagates through the fifth polarization-maintaining optical fiber 58, and is input to the third input / output terminal 52-3 of the third polarization separation / combination module 52. Then, the light is output from the fourth input / output end 52-4 on the side facing the third input / output end 52-3 and propagates through the sixth polarization-maintaining optical fiber 60.

  On the other hand, the second signal light is input to the fourth input / output end 52-4 of the third polarization separation / combination module 52 and is opposite to the fourth input / output end 52-4. Is transmitted to the fifth polarization plane-maintaining optical fiber 58 and propagates through the fifth polarization plane-maintaining optical fiber 58, and the third polarization plane conversion unit 56 rotates the plane of polarization by 90 ° to preserve the fourth polarization plane. The light propagates through the optical fiber 54 and is input to the second input / output terminal 52-2 of the third polarization separation / combination module 52. Then, it is output from the third input / output terminal 52-3 of the third polarization separation / combination module 52 and propagates again through the fifth polarization plane preserving optical fiber 58, and the polarization plane again becomes 90 ° at the third polarization plane converter 56. Rotating and propagating through the fourth polarization-maintaining optical fiber 54, input from the second input / output end 52-2 of the third polarization separation / combination module 52 and facing the second input / output end 52-2 The light is output from the first input / output terminal 52-1 and is input to the third polarization-maintaining optical fiber 22 and propagates through the third polarization-maintaining optical fiber 22.

  The contribution of the control light to the phase change of the signal light is brought about by the fourth polarization-maintaining optical fiber 54 and the fifth polarization-maintaining optical fiber 58 in the optical nonlinear controller 50. That is, the phase change of the signal light becomes more prominent as the lengths of the fourth polarization-maintaining optical fiber 54 and the fifth polarization-maintaining optical fiber 58 are longer.

  As described above, the first and second signal lights propagate through the fourth polarization plane preserving optical fiber 54 and the fifth polarization plane preserving optical fiber 58 twice, respectively. This is realized by the fact that the polarization plane is rotated by 90 ° in the third polarization plane converter 56 and the polarization plane selectivity of the polarization plane selective reflection surface of the third polarization separation / combination module 52. to cause. The reason for this is as follows.

First, consider the first signal light. First signal light output from the second output terminal 52-2 of the third polarization splitting-combining module 52, by passing through the third polarization plane conversion unit 56 propagates through the path L ₄ as TM polarization , Converted to TE polarized wave, propagated through the path L ₅ , and input to the third input / output terminal 52-3 of the third polarization separation / combination module 52.

  When TM polarization is input to the third polarization separation / combination module 52, the signal travels straight through the third polarization separation / combination module 52 and is output to the input / output terminal opposite to the input side. On the other hand, when TE polarization is input to the third polarization separation / combination module 52, the reflected light is reflected by the reflection surface 52R having polarization plane selectivity existing inside the third polarization separation / combination module 52. Is output from the input / output terminal on the side facing the.

Therefore, the first signal light input to the third input / output terminal 52-3 of the third polarization separation / combination module 52 as the TE polarization is the second input / output terminal 52-2 of the third polarization separation / combination module 52. Is propagated through the path L ₄ as TE polarization and passes through the third polarization plane converter 56, converted to TM polarization, propagated through the path L ₅ , and again separated into the third polarization. The data is input to the third input / output terminal 52-3 of the synthesis module 52. This time, since it is input to the third input / output terminal 52-3 of the third polarization separation / combination module 52 as the TM polarization, the fourth input / output terminal 52- on the side facing the third input / output terminal 52-3 is used. 4 is input to the sixth polarization-maintaining optical fiber 60 as TM polarization.

On the other hand, the path of the second signal light is as follows. Second signal light output from the third output terminal 52-3 of the third polarization splitting-combining module 52, by passing through the third polarization plane conversion unit 56 propagates through the path L ₅ as TM polarization , are converted to TE polarization is input as TE polarization to the second input terminal 52-2 of the third polarization splitting-combining module 52 propagates through the path L _4.

The second signal light input as the TE polarization to the second input / output terminal 52-2 of the third polarization separation / combination module 52 is output from the third input / output terminal 52-3 of the third polarization separation / combination module 52. The path L ₅ is propagated as the TE polarization and passes through the third polarization plane conversion unit 56, so that it is converted to the TM polarization, and the path L ₄ is propagated to the third polarization separation / synthesis module again. 52 is input to the second input / output terminal 52-2. This time, since the TM polarization is input to the second input / output terminal 52-2 of the third polarization separation / combination module 52, the first input / output terminal 52- facing the second input / output terminal 52-2 1 is output as TM polarization and input to the third polarization plane preserving optical fiber 22.

That is, both the first signal light and the second signal light are output from the third polarization separation / combination module 52 to the sixth polarization plane preserving optical fiber 60 and the third polarization plane preserving optical fiber 22 as TM polarization, respectively. That is, by providing the optical nonlinear controller 50, the first and second signal lights propagate along a path that is long enough to propagate through the fourth and fifth polarization-maintaining optical fibers 54 and 58. The polarization planes of the first and second optical signals input / output to / from the nonlinear controller 50 remain TM polarized. In other words, it can be considered to correspond to the path L ₃ of the optical switch of the first embodiment shown in FIG. 1 are replaced by optical nonlinear control unit 50.

Assume that the slow axis effective refractive index of the fourth and fifth polarization-maintaining optical fibers 54 and 58 is n _s , the fast axis effective refractive index is n _f, and the direction of the slow axis matches the polarization plane of the TM polarization. When the first signal light is output as TM polarization from the second input / output terminal 52-2 of the third polarization separation / combination module 52, the third input / output terminal 52- The optical path length until it is input to TM as TM polarization is added in the order of the path of propagation of the first signal light, and is given by the following equation (4).
n _s l ₄ + n _f l ₅ + n _f l ₄ + n _s l ₅ (4)
Here, l ₄ and l ₅ are the lengths of the paths L ₄ and L ₅ , respectively.

That is, the first signal light is output from the second output terminal 52-2 of the third polarization splitting-combining module 52, the propagation path L ₄ as TM polarization was (optical path length = n _s l _4), the path L ₅ propagates as TE polarization (optical path length = n _f l ₅ ), path L ₄ propagates as TE polarization (optical path length = n _f l ₄ ), and path L ₅ propagates as TM polarization (optical path) Length = n _s l ₅ ) The total optical path length is given by n _s l ₄ + n _f l ₅ + n _f l ₄ + n _s l ₅ . Therefore, the path L ₄ (fourth polarization-maintaining optical fiber 54) and the path L ₅ (fifth polarization-maintaining optical fiber 58) are propagated twice as a TM polarization and a TE polarization, respectively, twice in total. I understand that

The second signal light can also be considered in the same way as the first signal light, and the second signal light is output as the TM polarization from the third input / output terminal 52-3 of the third polarization separation / combination module 52. The optical path length until it is input as the TM polarization to the second input / output terminal 52-2 of the third polarization separation / combination module 52 is similarly added in the order of the propagation path of the second signal light. Given in (5).
n _s l ₅ + n _f l ₄ + n _f l ₅ + n _s l ₄ (5)
The control light follows the same path as the first signal light and propagates in the same direction as the first signal light.

  Comparing the above formulas (4) and (5), the first term, the second term, the third term, the fourth term in the formula (4) and the fourth term, the third term, the second term, the first term in the formula (5) and Are equal to each other. That is, it can be seen that the first signal light and the second signal light propagate through the same optical path length.

That is, when the control light is not input, the first signal light and the second signal light are combined in the third polarization separation / combination module 52 in the same phase. When the control light is input, the optical Kerr effect appears in the optical fiber loop and the refractive index changes. Therefore, the first signal light and the second signal light propagate respectively clockwise and counterclockwise in the optical fiber loop constituted by the paths L ₄ and L ₅ , respectively, and again the third polarization separation / synthesis module When they are combined at 52, the phases of both are out of phase.

If the intensity of the control light is adjusted so that the phase shift amount φ is equal to π, or the lengths of the fourth and fifth polarization-maintaining optical fibers 54 and 58 are adjusted, the optical switch as described above Is realized. The phase shift amount φ is proportional to the total length of the optical fiber loop composed of the paths L ₄ and L ₅ as given by the above equation (1). That is, the optical switch of the present invention, the path L ₄ and the path L ₅ each one by one as the TM polarization and TE polarization, since it is configured to propagate a total twice a substantially path L ₄ pathway length fraction only of the L _5, equivalent to the optical Kerr effect are taken long path expressed.

Here, the lengths of the path L ₄ and the path L ₅ are described as being set to be much longer than those of the other paths L ₁ , L ₂ , L ₃ , L ₆ and L ₇ . In other words, it has been explained that the polarization plane preserving optical fiber constituting the path L ₄ and the path L ₅ contributes mainly to the expression of the phase shift amount φ based on the optical Kerr effect, but it is limited to this. There is no. According to the equation (1), the phase shift amount φ is proportional to the product of the length of the optical fiber constituting the optical fiber loop and the intensity of the control light. Therefore, according to the optical switch of the present invention, Compared with the optical switch, the length of the polarization-maintaining optical fiber corresponding to the polarization-maintaining optical fiber constituted by the path L ₄ and the path L ₅ that mainly contribute to the expression of the phase shift amount φ based on the optical Kerr effect is half. Will be enough.

That is, the ability to shorten the length of the polarization-maintaining optical fiber formed by the path L ₄ and the path L ₅ is effective for making the optical switch compact. In addition, since the length of the polarization-maintaining optical fiber formed by the path L ₄ and the path L ₅ can be shortened, it is less affected by the ambient temperature and the like, which is effective for stabilizing the operation state of the optical switch.

<Third embodiment>
The structure and operation of the optical switch of the third embodiment will be described with reference to FIG.

(Construction)
This optical switch includes a first polarization separation / combination module 10, a second polarization separation / combination module 18, a first polarization plane preserving optical fiber 12, a second polarization plane preserving optical fiber 16, and a first polarization plane conversion. Unit 14, a third polarization-maintaining optical fiber 22, a seventh polarization-maintaining optical fiber 82, and an optical nonlinear controller 70. In addition, it is preferable to include an optical bandpass filter 28 and an optical circulator 30. In the following description, the description of the parts common to the optical switch of the second embodiment is omitted.

  In the optical nonlinear controller 70, the third polarization-maintaining optical fiber 22 and the seventh polarization-maintaining optical fiber 82 are coupled, and the first input / output end 72-1, the second input / output end 72-2, A third polarization separation / combination module 72 having three input / output ends 72-3, a first input / output end 80-1, a second input / output end 80-2, and a third input / output end 80-3 A four-polarization separation / combination module 80 and fourth to sixth polarization-maintaining optical fibers 84, 74, and 78 are provided. A second polarization plane converter 76 is provided.

  The fourth polarization plane preserving optical fiber 84 has one end coupled to the second input / output end 72-2 of the third polarization separation / combination module 72 opposite to the first input / output end 72-1, so that the fourth polarization The other end of the wave separation / synthesis module 80 is coupled to the second input / output end 80-2 facing the first input / output end 80-1. The fifth polarization-preserving optical fiber 74 has a second polarization plane converter 76 that rotates the polarization plane by 90 ° at one end, and the other end is connected to the first input / output end 80-1 of the fourth polarization separation / combination module 80. Are combined. The sixth polarization plane preserving optical fiber 78 has a second polarization plane conversion unit 76 that rotates the plane of polarization by 90 ° at one end, and the other end is connected to the third input / output end 72-3 of the third polarization separation / combination module 72. Are combined.

  The other end of the third polarization-maintaining optical fiber 22 is coupled to the first input / output terminal 72-1 of the third polarization separation / combination module 72, and the third input of the fourth polarization separation / combination module 80 is connected. The other end of the seventh polarization-maintaining optical fiber 82 is coupled to the output end 80-3.

  The characteristics of the reflection and transmission of the polarization plane selective reflection surfaces of the third and fourth polarization separation / combination modules 72 and 80 are the polarization plane selection of the polarization separation / combination module used in the second embodiment described above. Since it is the same as the reflecting surface, its detailed description is omitted.

(Operation)
With reference to FIG. 6, the operation principle of the optical switch of the third embodiment will be described. According to the optical switch of the third embodiment, the first signal light propagated in the CW direction through the third polarization-maintaining optical fiber 22 is the first signal of the third polarization separation / combination module 72 of the optical nonlinear controller 70. The second signal light that is input to the input / output end 72-1 and propagates in the CCW direction through the seventh polarization-maintaining optical fiber 82 is the third input / output end of the fourth polarization separation / combination module 80 of the optical nonlinear controller 70. Input to 80-3.

  The first signal light propagates through the third polarization-maintaining optical fiber 22 as TM polarization, and then is input to the optical nonlinear controller 70. That is, it is input from the first input / output end 72-1 of the third polarization separation / combination module 72, output from the second input / output end 72-2, and input to the fourth polarization plane preserving optical fiber 84, and the fourth The fifth polarization-maintaining optical fiber 74 propagates through the polarization-maintaining optical fiber 84, is input from the second input / output end 80-2 of the fourth polarization separation / combination module 80, and is output from the first input / output end 80-1. Is input.

  Then, the second polarization plane maintaining unit 76 is propagated through the fifth polarization plane preserving optical fiber 74 as TM polarization and provided at one end of the fifth polarization plane preserving optical fiber 74, the position indicated by the point B in FIG. , The plane of polarization is rotated by 90 °, propagates through the sixth polarization plane preserving optical fiber 78 as TE polarization, and is input to the third input / output terminal 72-3 of the third polarization separation / combination module 72. And, since it was input to the third input / output terminal 72-3 of the third polarization separation / combination module 72 as TE polarization, it was output from the second input / output terminal 72-2 of the third polarization separation / combination module 72. The fourth polarization plane preserving optical fiber 62 is propagated again as the TE polarization and input to the second input / output terminal 80-2 of the fourth polarization separation / combination module 80. Since it has been input from the second input / output end 80-2 as TE polarization, it is output from the third input / output end 80-3 and input to the seventh polarization-maintaining optical fiber 82 as TE polarization.

  On the other hand, the second signal light propagates through the seventh polarization-maintaining optical fiber 82 as TE polarized light, and then is input to the optical nonlinear controller 70. That is, the TE polarized wave is input from the third output terminal 80-3 of the fourth wave separation / combination module 80, output from the second input / output terminal 80-2, and input to the fourth polarization-preserving optical fiber 84 to obtain the fourth polarization. The sixth polarization-maintaining optical fiber propagates through the polarization-maintaining optical fiber 84, is input from the second input / output end 72-2 of the third polarization separation / combination module 72, and is output from the third input / output end 72-3. Input to 78.

  Then, the second polarization plane conversion unit 76 provided at one end of the sixth polarization plane maintaining optical fiber 78 and propagating through the sixth polarization plane maintaining optical fiber 78, the position indicated by the point B in FIG. Is rotated by 90 ° and input to the fifth polarization-maintaining optical fiber 74 as TM polarization, propagates through the fifth polarization-maintaining optical fiber 74, and then enters the first input of the fourth polarization separation / combination module 80. Input to the output terminal 80-1. Since it is input to the first input / output end 80-1 as TM polarization, it is output from the second input / output end 80-2 and input to the fourth polarization plane preserving optical fiber 84, and is input to the fourth polarization plane as TM polarization. Propagated again through the wavefront-preserving optical fiber 84, input to the second input / output end 72-2 of the third polarization separation / combination module 72, and output from the first input / output end 72-1 as TM polarization Input to the polarization plane preserving optical fiber 22.

  That is, the first signal light is input to the seventh polarization-maintaining optical fiber 82 as TE polarization, and the second signal light is input to the third polarization-maintaining optical fiber 22 as TM polarization. Therefore, by providing the optical nonlinear controller 70, the first and second signal lights have the effect of propagating along a path that is long enough to propagate twice through the fourth polarization plane preserving optical fiber 84, and FIG. When compared with the optical switch of the first embodiment shown, the following correspondence is established. The fourth polarization-maintaining optical fiber 26 and the third polarization-maintaining optical fiber 22 of the optical switch of the first embodiment shown in FIG. 1 are the same as those of the optical switch of the third embodiment shown in FIG. The seventh polarization-maintaining optical fiber 82 and the third polarization-maintaining optical fiber 22 correspond to each other. In other words, a part of the fourth polarization-maintaining optical fiber 26 and the third polarization-maintaining optical fiber 22 on both sides sandwiching the second polarization plane conversion unit 24 of the optical switch of the first embodiment shown in FIG. It can be considered that this corresponds to the replacement with the non-linear controller 70.

  The optical switch according to the third embodiment is different from the optical switch according to the second embodiment in the following points. That is, the polarization separation / combination module 52 corresponding to the polarization separation / combination module 52 in the optical switch of the second embodiment has two locations, the third and fourth polarizations, in the optical switch of the third embodiment. Since the configuration is set in the wave separation / combination modules 72 and 80, the selectivity of the polarization plane of the switched signal light is doubled, and compared with the optical switch of the second embodiment, the polarization crosstalk component Removal is performed more effectively. Moreover, even when the polarization plane selectivity of the polarization separation / combination module is incomplete, the polarization plane selectivity by the polarization separation / combination module is substantially improved.

  The first and second signal lights are each propagated twice through the fourth polarization-maintaining optical fiber 84, and this is realized because the polarization plane is changed by the second polarization plane converter 76. This is due to the 90 ° rotation and the polarization plane selectivity of the third polarization separation / combination module 72 and the fourth polarization separation / combination module 80. The reason for this is as follows.

  The first signal light propagates in the third polarization plane preserving optical fiber 22 as TM polarization, and propagates in the fourth polarization plane preserving optical fiber 84 as TM polarization via the third polarization separation / combination module 72. Then, the fifth polarization plane preserving optical fiber 74 is propagated as TM polarization through the fourth polarization separation / combination module 80, passes through the second polarization plane conversion unit 76, is converted into TE polarization, and is converted into the TE polarization. It propagates through the 6 polarization maintaining optical fiber 78.

  Then, since it is input to the third input / output end 72-3 of the third polarization separation / combination module 72 as the TE polarized wave, it is output from the second input / output end 72-2 and again the fourth polarization plane preserving optical fiber 84. Will propagate as TE polarization. This time, since the TE polarization is input to the second input / output end 80-2 of the fourth polarization splitting / combining module 80, it is output from the third input / output end 80-3, and the seventh polarization plane-maintaining optical fiber 82 is output. Will propagate as TE polarization.

  That is, the fourth polarization plane preserving optical fiber 84 is propagated first as TM polarization and the second time as TE polarization, and the fourth polarization plane preserving optical fiber 84 is propagated twice in total. On the other hand, the same applies to the second signal light.The fourth polarization plane preserving optical fiber 84 is propagated first as the TE polarization and the second time as the TM polarization. Will propagate once.

As in the case of the optical switch of the second embodiment, the effective refractive index of the slow axis of the fourth to sixth polarization-maintaining optical fibers is n _s , the effective refractive index of the fast axis is n _f , and the slow axis When the direction coincides with the polarization plane of the TM polarization, the first signal light is output from the second input / output terminal 72-2 of the third polarization separation / combination module 72 and stored in the fourth to sixth polarization planes. The optical path length from the propagation through the optical fiber to the second input / output end 80-2 of the fourth polarization separation / combination module 80 is obtained as follows. The first signal light is output from the second input / output terminal 72-2 of the third polarization separation / combination module 72 as TM polarization.

The length of the fourth polarization-maintaining optical fiber 84 is l ₄ (path L ₄ ), the length of the fifth polarization-maintaining optical fiber 74 is l ₅ (path L ₅ ), and the length of the sixth polarization-maintaining optical fiber 78 is If the length is l ₆ (path L ₆ ), the optical path length is given by the following equation (6) by adding in order of the path along which the first signal light propagates.
n _s l ₄ + n _s l ₅ + n _f l ₆ + n _f l ₄ (6)

That is, since the first signal light propagates through the fourth polarization-maintaining optical fiber 84 as TM polarization, the optical path length is n _s l ₄ and the fifth polarization-maintaining optical fiber 74 also propagates as TM polarization. Therefore, the optical path length is n _s l ₅ . Here, since the first signal light propagates through the sixth polarization plane preserving optical fiber 78 as a TE polarization whose polarization plane is rotated by 90 ° in the second polarization plane conversion unit 76, the optical path length is n _f l ₆ is there. Then, when again propagating a fourth polarization maintaining optical fiber 84, the optical path length so propagated as TE polarization is n _f l _4.

On the other hand, the second signal light is output from the second input / output terminal 80-2 of the fourth polarization beam splitting / combining module 80 as TE polarization. The second signal light is output from the second input / output end 80-2 of the fourth polarization separation / combination module 80, propagates through the fourth to sixth polarization-preserving optical fibers, and passes through the third polarization separation / combination module 72. The optical path length until reaching the second input / output terminal 72-2 is added by the order of the paths through which the second signal light propagates, and is given by the following equation (7).
n _f l ₄ + n _f l ₆ + n _s l ₅ + n _s l ₄ (7)
Note that also in the optical switch of the third embodiment, the control light follows a path that propagates in the same direction as the first signal light.

  Comparing the above formulas (6) and (7), the first term, the second term, the third term, the fourth term in the formula (6) and the fourth term, the third term, the second term, the first term in the formula (7) and Are equal to each other. That is, it can be seen that the first signal light and the second signal light propagate through the same optical path length.

  That is, when phase intermodulation based on control light is given to the first signal light, the first signal light reaches the second input / output terminal 72-2 of the third polarization separation / combination module 72, The transfer is delayed as compared with the case where it is not affected by the control light. Therefore, when the first signal light and the second signal light are combined by the second polarization splitting / combining module 18 via the optical nonlinear controller 70, the phases of both are shifted.

An optical switch can be realized by adjusting the intensity of the control light or adjusting the length of the fourth polarization-preserving optical fiber 84 so that the phase shift amount φ is equal to π. As the phase shift amount φ given by the above equation (1), proportional to the length of the fourth polarization maintaining optical fiber 84 composed of a path L _4. That is, the third embodiment of the optical switch, each respective one path L ₄ as TM polarization and TE polarization, since it is configured to propagate a total twice a substantially path L ₄ This is equivalent to the length of the path in which the optical Kerr effect is developed.

Here, the length of the path L ₄ has been described as being set to be much longer than those of the other paths L ₁ , L ₂ , L ₃ , L ₅ , L ₆ and L ₇ . That is, the are mainly contributes to the expression of the phase shift amount φ based on the optical Kerr effect was described as a fourth polarization maintaining optical fiber 84 constituting the path L _4, it is limited to this Absent. According to the equation (1), the phase shift amount φ is proportional to the product of the length of the optical fiber constituting the optical fiber loop and the intensity of the control light. Therefore, according to the optical switch of the third embodiment, For example, as compared with the conventional optical switch, the length of the polarization-maintaining optical fiber corresponding to the fourth polarization-maintaining optical fiber 84 formed by the path L ₄ that mainly contributes to the expression of the phase shift amount φ based on the optical Kerr effect. That's half.

That is, being able to shorten the length of the fourth polarization maintaining optical fiber 84 constituting the path L ₄ are, is effective in downsizing of the optical switch. Moreover, since the length of the polarization-maintaining optical fiber constituting the path L ₄ can be shortened, it is less susceptible to the influence of the ambient temperature and the like, which is effective for stabilizing the operation state of the optical switch.

1 is a schematic configuration diagram of an optical switch according to a first embodiment of the present invention. It is a schematic sectional drawing of a polarization-maintaining optical fiber. It is a figure where it uses for description of the structure of a polarization plane conversion part. FIG. 5 is a diagram for explaining the operation of the optical switch according to the first embodiment of the present invention. FIG. 5 is a schematic configuration diagram of an optical switch according to a second embodiment. FIG. 6 is a schematic configuration diagram of an optical switch according to a third embodiment.

Explanation of symbols

10: 1st polarization separation / synthesis module
12: First polarization-maintaining optical fiber
14: First polarization plane converter
16: Second polarization-maintaining optical fiber
18: Second polarization separation / synthesis module
20: Optical coupler
22: Third polarization preserving optical fiber
24, 62, 76: Second polarization plane converter
26, 54, 84: Fourth polarization-maintaining optical fiber
27: Optical fiber for output
28: Optical bandpass filter
29: Optical fiber for transmitted light output
30: Optical circulator
32: Input optical fiber
36: Optical fiber for reflected light output
50, 70: Optical nonlinear controller
52, 72: Third polarization separation and synthesis module
58, 74: 5th polarization-maintaining optical fiber
56: Third polarization plane converter
60, 78: 6th polarization-maintaining optical fiber
64, 82: 7th polarization-maintaining optical fiber
80: Fourth polarization separation / synthesis module
140: Clad
142: Core
144: Stress application part
170, 172: PANDA type optical fiber

Claims (6)

A first input / output end for inputting signal light, a second input / output end on the side opposite to the first input / output end for coupling one end of the first polarization-maintaining optical fiber, and a third for outputting switched signal light A first polarization separation / synthesis module having an input / output end;
A first input / output end for coupling the other end of the second polarization-maintaining optical fiber, a second input / output end for coupling one end of the third polarization-maintaining optical fiber to the side facing the first input / output end, and a fourth A second polarization separation / combination module comprising a third input / output end for coupling one end of a polarization-maintaining optical fiber, and a fourth input / output end for outputting a polarization crosstalk component on the side opposite to the third input / output end When,
The first polarization plane preserving optical fiber having one end coupled to the second input / output end of the first polarization separation / combination module;
The second polarization plane preserving optical fiber having the other end coupled to the first input / output end of the second polarization separation / combination module;
A first polarization plane converter that connects the other end of the first polarization-maintaining optical fiber and one end of the second polarization-maintaining optical fiber;
One end coupled to the second input / output end of the second polarization separation / combination module, the third polarization plane preserving optical fiber comprising an optical coupler,
The fourth polarization-preserving optical fiber having one end coupled to the third input / output end of the second polarization separation / combination module;
An optical switch comprising: a second polarization plane conversion unit that connects the other end of the third polarization-maintaining optical fiber and the other end of the fourth polarization-maintaining optical fiber.   2. The optical switch according to claim 1, wherein the first polarization plane converting unit has an optical axis of the first polarization plane preserving optical fiber and an optical axis of the second polarization plane preserving optical fiber of 45 ° with respect to each other. An optical switch formed by fusing at an angle.   2. The optical switch according to claim 1, wherein the second polarization plane converting unit is configured such that an optical axis of the third polarization plane preserving optical fiber and an optical axis of the fourth polarization plane preserving optical fiber are 90 ° to each other. An optical switch formed by fusing at an angle. 2. The optical switch according to claim 1, wherein the first polarization plane converting unit has an optical axis of the first polarization plane preserving optical fiber and an optical axis of the second polarization plane preserving optical fiber of 45 °. It is formed by fusing at an angle,
The second polarization plane converting unit is formed by fusing the optical axis of the third polarization plane preserving optical fiber and the optical axis of the fourth polarization plane preserving optical fiber at an angle of 90 ° with each other. An optical switch characterized by having   5. The optical switch according to claim 1, wherein one end of an output optical fiber is coupled to the third input / output end of the first polarization separation / combination module. An optical switch, wherein an optical bandpass filter is connected to the other end of the optical fiber. 5. The optical switch according to claim 1, wherein one end of an input optical fiber is coupled to the first input / output end of the first polarization separation / combination module. An optical switch, characterized in that an optical circulator is connected to the other end of the optical fiber.

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